1. Field of the Invention
The present invention relates to a boiling heat transfer tube which is incorporated in an flooded evaporator of a vapor compression refrigerating machine, such as a centrifugal type chiller, a screw type chiller, immersed in a liquid refrigerant (for example, freon, liquid nitrogen or the like) and used for heating and boiling a liquid refrigerant and particularly, to a boiling heat transfer tube which is improved on its heat transfer performance for a low density refrigerant.
2. Related Prior Art
Several kinds in shape of heat transfer surface have heretofore proposed as boiling heat transfer tubes of this kind. For example, as disclosed in the published Examined Japanese Patent Application Nos. Sho 53-25379 and Hei 4-78917, fins are formed on the outer surface of the tube, cuts to form holes are given in the tip of each fin and the tips of fins are turned down to form useful cavities for boiling heat transfer.
Besides, for example, as described in the published Examined Japanese Patent Application No. Sho 64-2878 and the published Unexamined Japanese Patent Application No. Hei 8-219674, after fins in a spiral fashion are formed on the outer surface of the tube, the tips of the fins are deformed by compression to form cavities in directions of a tube periphery and a tube axis and gaps of 0.13 mm or less in width are provided for communication between the cavities and the outside.
Furthermore, for example, as described in the published Unexamined Japanese Patent Application Nos. Hei 4-236097 and Hei 7-151485, in order to improve a heat transfer performance, not only is boiling in a cavity accelerated but turbulence of a liquid refrigerant and gasified refrigerant on the tube outer surface are also encouraged.
While these heat transfer tubes are improved in heat transfer performance when a refrigerant, such as trichlorofluoromethane, chlorodifluoromethane, or 1, 1-dichloro-2,2,trifluoroethane, is used, there has been a problem, when a low density refrigerant, such as 1,1,2-tetrafluoroehtane is used, that a heat transfer performance is reduced since a conventional heat transfer tube has a small opening (a gap) where a cavity and the outside are communicated, which resists flowing-in of a liquid refrigerant into the cavity, and makes a space in the cavity dried.
In order to avoid this problem, a method can be considered that a quantity of a liquid refrigerant charged in a flooded evaporator is increased, but it has a fault that a charge cost of a liquid refrigerant is increased and in addition, a requirement arises that a volume of an heat exchanger is larger, which in turn makes a cost further increased.